Phosphoric acid is an important commodity chemical product. Its primary use is in the production of fertilizers but there are numerous additional applications for phosphoric acid with varied requirements for purity.
Phosphoric acid is commonly manufactured using what is known as the wet process method. In this method, an aqueous mixture is prepared which contains an appropriate ore comprising a substantial amount of tricalcium phosphate rock (e.g. by grinding the ore in an aqueous solution to produce an aqueous slurry, or merely combining ore as obtained with water, etc.). Sulfuric acid is added thereto which digests the rock and produces a solution comprising phosphoric acid and other species depending on what other minerals were present in the ore. Calcium sulfate precipitate is also produced and this precipitate is subsequently filtered and separated from the phosphoric acid solution. Depending on the grade (purity) of the phosphoric acid desired, additional process steps may then be required to remove unwanted elements and/or compounds. Preferably however, the complexity and costs associated with any such additional process steps are kept to a minimum.
In the production of international grade granular fertilizers, such as diammonium phosphate (DAP) and monoammonium phosphate (MAP) fertilizers, it is important to achieve certain impurity limits on iron and aluminum content for processing and handling purposes. Further, it is often necessary to achieve certain limits on other elements for environmental reasons (e.g. cadmium).
Ore deposits are frequently characterized by a quantity known as the minor element ratio (MER) which is defined as ratio of the sum of the weight percentages of Fe2O3, Al2O3, and MgO present in the ore to that of the P2O5 present in the ore. It is often considered that the MER of an ore must be less than about 0.1 for the ore to be suitable for the production of DAP. However, many ore deposits which comprise substantial amounts of elemental iron (e.g. greater than 1.2% by weight) are now being identified and considered for use. Thus, methods for simply and economically removing iron are desired in manufacturing phosphoric acid from such ores. Along with undesirable impurities, the ore deposits used for phosphoric acid production may also contain other species with significant commercial value, e.g. rare earth elements. Where economically possible, it is thus desirable to recover this valuable content from the ore.
Numerous techniques have been considered and reported on in the art for producing phosphoric acid and removing impurities and separating other valuable content therefrom. For instance, U.S. Pat. No. 4,108,957 discloses an alternative method to the wet process method for producing phosphoric acid. Crushed phosphate rock is mixed with phosphoric acid to form a slurry and the slurry is then heated to produce calcium monophosphate. Thereafter, oxalic acid is added to the slurry to precipitate the calcium therein as calcium oxalate which is separated. It was noted that the addition of oxalic acid provides a method for producing insoluble forms not only of the calcium values in phosphate rock but also of the metallic impurities (particularly uranium).
However, there is still a need to develop improved techniques for producing phosphoric acid from the wide range of ore deposits found throughout the world and particularly to develop improved techniques for removing impurities, such as iron, and/or to recover, rather than waste, other valuable content in the ore. The present invention addresses these needs and provides other benefits as disclosed below